Soybeans are often called the meat from the field in view of the fact that they comprise foodstuff materials for providing high-quality vegetable protein, and have recently been used to an increasing extent.
A problem which arises when using soybeans as foodstuff materials is their own odor, referred to as the soybean flavor, which is an obstacle to the satisfactory use of soybeans as foodstuff materials.
It is well known in the art that the soybean odor results from a bad-smelling substance produced by the interaction between the enzyme lipoxigenase and unsaturated fatty acids in the presence of oxygen during the grinding of soybeans. This enzymatic action is rapid. Once the bad-smelling substance is produced, it is extremely difficult to remove the adverse influence thereof.
While lipoxigenase can be inactivated by heating, over-heating deteriorates the soybean protein, leading to a marked decrease in the value of the foodstuff materials produced therefrom.
When treating soybeans, therefore, it is required that lipoxigenase be inactivated to a sufficient degree, while keeping the NSI at a high level. However, it is not always easy to determine the heating conditions for simultaneously achieving both of these objects.
It is well known to treat soybeans by exposure to, e.g., an atmosphere of at least 1 atm and at an elevated temperature of as high as 100.degree. C. or more for the inactivation of the lipoxigenase. (See Japanese Patent Publication No. 48-19946, Japanese Patent laid-Open Publication Nos. 48-82052 and 53-29951 as well as Japanese Patent Publication No. 59-44022). Under these conditions, however, the soybean protein deteriorates so easily that the NSI may decrease.
Alternatively, soybeans have been heated under mechanical vibration while being allowed to fall in a tower or column. (See Japanese Patent Laid-Open Publication No. 53-142555). However, insufficient inactivation of lipoxigenase takes place due to the short-pass of the beans during processing, so that the remaining lipoxigenase results in the occurrence of an offensive odor by enzymatic action and inevitably causes the foodstuffs in which such beans are used to be unpalatable.